5. Formatos
5.3. Formatos de párrafo
In this work, a series of bench-scale BFB tests were carried out to characterize the formation and sulfation behaviors of KCl and KBr. Spruce bark was used as the base fuel - serving as a source of alkali metals (mainly K, some Na). HCl and HBr gases were fed with the fluidization air to simulate co-firing of a halide-rich fuel with bark, thereby forming alkali halide aerosols in the flue gas.
It was found that the addition of HBr or HCl greatly increased the release of alkali metals (K, Na) from the original fuel during combustion. The promoting effect of HBr was more obvious than that of HCl. Further, HBr was found to have a clearly higher tendency to form alkali halides than HCl.
In the bench-scale BFB tests, SO2 and elemental S powder were used as additives to convert the
alkali halide aerosols to less corrosive alkali sulfates. The sulfation extent with S was somewhat lower than with SO2, but generally on the same level. The results indicated a higher tendency for
sulfation of alkali chloride than of alkali bromide. These results were also supported by thermodynamic calculations.
A series of laboratory tests were carried out to investigate the high-temperature corrosion behaviors of three different superheater steels (10CrMo9-10, AISI 347 and Sanicro 28) exposed to potassium halides in ambient air and wet air containing 30% H2O. The influence of H2O and
O2 on the high temperature corrosion of steels with and without KCl in three gas atmospheres
(2% H2O-30% O2-N2, 2% H2O-2% O2-N2 and 30% H2O-2% O2-N2) was also studied. The most
important results are summarized below: Ambient air:
At relatively low temperatures (≤ 550 °C), the corrosivity of KBr and KF are similar to KCl. At 600 °C, KF showed much stronger corrosivity than KBr and KCl. When exposed to KBr or KF,
10CrMo9-10 could be durable up to 450 °C; AISI 347 and Sanicro 28 could be durable up to 550 °C.
Wet air (containing 30% H2O):
At 450 °C, the influence of water vapor was not obvious. At 550 °C, the influence of water vapor became significant in some cases, but the trend was not consistent. At 550 °C, 10CrMo9-10 suffered from much more corrosion (oxide layer thickness of 233 µm) when exposed to KBr than when exposed to KF and KCl. At 550 °C, local deep pitting corrosion occurred on AISI 347 and Sanicro 28 when exposed to KF. The deepest pitting depth was about 40-50 µm. Some formation of K2CrO4 was observed in the oxide layer. At 550 °C, low corrosion was observed on AISI 347
and Sanicro 28 when exposed to KBr and KCl. Gas atmospheres with different H2O/O2 ratios:
The corrosion of 10CrMo9-10 without salt increased with increasing H2O (decreasing O2), while
the corrosion of 10CrMo9-10 with KCl decreased with increasing H2O (decreasing O2). In the
“O2-rich” atmosphere, the presence of KCl significantly increased the corrosion of 10CrMo9-10.
However, in the “H2O-rich” atmosphere, the presence of KCl did not result in any significant
differences on the corrosion of 10CrMo9-10. It is suggested that in the “H2O-rich” atmosphere,
volatile CrO2(OH)2 is mainly formed, while in the “O2-rich” atmosphere and in the presence of
KCl, K2CrO4 is mainly formed.
In the tests with no salt at 500-600 °C, no corrosion occurred on AISI 347 and Sanicro 28. In the tests with KCl at 500-600 °C, low corrosion occurred on these two steels with oxide layer thicknesses mostly under 20 µm. The influence of different H2O/O2 ratios on the corrosion of
these two steels showed no consistent trend.
Considering both the results from the BFB tests and the laboratory corrosion tests, if waste fuels with equal amounts of Br or Cl were to be combusted, the corrosion damage of superheaters would be expected to be higher in the bromine case. The possible formation of potassium fluoride in BFB combustion conditions has not yet been studied, which needs to be investigated in the
future work. Also, more detailed investigations on the effect of H2O on high-temperature
corrosion needs attention, especially in the presence of possible ash compounds. Furthermore, also other salts, such as lead- and zinc bromides and/or fluorides may become important in the future due to the increasing use of demolition wood as a fuel for power and heat production.
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